There are around 25 native species of earthworm in the UK. Three of these varieties cause much of the casting found on lawns. The casting leads to smearing when walked on or mowed. This smearing leads to capping and in turn bare patches that grass shoots cannot penetrate. These bare areas also provide a great seed bed for moss and weeds.
There is now no chemical treatment available to suppress the surface feeding worms. If you only have a few casts and conditions are dry it may be enough to brush the casts with a beesom broom to disperse them before mowing. Alternatively, you can use a metal rake with the teeth facing upwards and move from side to side to disperse the casts.